1. Field of the Invention
This invention relates to the recovery of soluble constituents from mixed solids by leaching and is particularly concerned with the recovery of alkali metal compounds from carbonaceous solids produced during the gasification of coal and in similar processes.
2. Description of the Prior Art
It is known that potassium carbonate and other alkali metal compounds catalyze the gasification of coal and similar carbonaceous solids and that such compounds can also be used to advantage in coal liquefaction, coal carbonization, and related processes. To secure the higher reaction rates made possible by the presence of alkali metal compounds in such processes, it has been suggested that bituminous coal, subbituminous coal, lignite, petroleum coke, oil shale, organic wastes, and similar materials be mixed or impregnated with potassium, cesium, sodium or lithium compounds before such materials are reacted with steam, hydrogen, oxygen or other agents at elevated temperatures. Studies have shown that a wide variety of different alkali metal compositions can be used for this purpose, including both organic and inorganic salts, oxides, hydroxides and the like. Because of their relatively low cast, the inorganic salts such as potassium carbonate are generally preferred.
Gasification and similar processes carried out in the presence of alkali metal catalysts at temperatures below about 2200.degree. F. generally result in the formation of chars and alkali metal catalyst residues. The chars include unconverted organic constituents of the coal or other feed material and inorganic constituents normally referred to as ash. The amount of ash present depends upon the composition of the feed material, the gasification or other reaction conditions employed, and the residence time of the particles in the reaction zone. It is common practice to withdraw a portion of the char from the reaction zone in order to eliminate the ash and keep it from building up within the gasifier or other reaction vessel. Elutriation methods and other techniques for separating char particles of relatively high ash content and returning particles of relatively low ash content to the reaction zone in order to improve the utilization of carbon in such processes have been suggested.
It has been proposed that alkali metal catalyst residues withdrawn from the reaction zone with the char during coal gasification and similar operations be recovered for reuse by leaching out the alkali metal constituents. Studies indicate that these constituents are generally present as carbonates or other water-soluble compounds which can be readily recovered by water-washing. Because of the limited extent to which alkali metal catalysts have been used, however, there has been relatively little work done in this area.